在 TikZ 中绘制路径图时遇到困难

Question

作为学习 TikZ 的初学者，我尝试了以下方法：

\documentclass[border=0.2cm]{standalone}
\usepackage{tikz}
\usetikzlibrary{automata, positioning}

\begin{document}
\begin{tikzpicture} [draw=black,
node distance = 3cm, 
on grid, 
auto,
every loop/.style={stealth-}]

\node (e1) [state] {$E$};
\node (c1) [state, right = of e1] {$C$};
\node (a1) [state, right = of c1] {$A$};
\node (t1) [state, below = of c1] {$T1$};
\node (a2) [state, right = of a1] {$A$};
\node (c2) [state, right = of a2] {$C$};
\node (e2) [state, right = of c2] {$E$};    
\node (t2) [state, below = of c2] {$T2$};

% Arrows
\path [-stealth, thick]
(e1) edge node {$e^2$}   (t1)
(c1) edge node {$c^2$}   (t1)
(a1) edge node {$h^2$}   (t1)
(e1) edge [loop above]  node {1}()
(c1) edge [loop above]  node {1}()
(a1) edge [loop above]  node {1}()
(e2) edge node {$e^2$}   (t2)
(c2) edge node {$c^2$}   (t2)
(a2) edge node {$h^2$}   (t2)
(e2) edge [loop above]  node {1}()
(c2) edge [loop above]  node {1}()
(a2) edge [loop above]  node {1}()
(a1) edge [bend left]  node {1}(a2)
(c1) edge [bend left]  node {1}(c2);
\end{tikzpicture}
\end{document}

目前的结果如下：

需要改进：

1. 将两条弯曲路径改为双向箭头的路径；
2. 将较长的弯曲路径稍微抬高一些，以避免重叠；
3. 将底部两个节点T1、T2由圆形改为矩形；
4. 水平对齐这六个标签 (e^2、c^2、h^2、...)。

谢谢！

Answer 1

干得好！

\documentclass[border=0.2cm]{standalone}
% https://tex.stackexchange.com/questions/678743/struggling-in-drawing-a-path-graph-in-tikz

\usepackage{tikz}
\usetikzlibrary{automata, positioning}

\begin{document}
\begin{tikzpicture} [draw=black,
node distance = 3cm, 
on grid, 
auto,
every loop/.style={stealth-},
myrect/.style={rectangle,draw,inner sep=8pt}]

\node (e1) [state] {$E$};
\node (c1) [state, right = of e1] {$C$};
\node (a1) [state, right = of c1] {$A$};
\node (t1) [myrect, below = of c1] {$T1$};
\node (a2) [state, right = of a1] {$A$};
\node (c2) [state, right = of a2] {$C$};
\node (e2) [state, right = of c2] {$E$};    
\node (t2) [myrect, below = of c2] {$T2$};

% Arrows
\path [-stealth, thick]
(e1) edge node[midway,right=3pt] {$e^2$}   (t1)
(c1) edge node[midway,right] {$c^2$}   (t1)
(a1) edge node[midway,right] {$h^2$}   (t1)
(e1) edge [loop above]  node {1}()
(c1) edge [loop above]  node {1}()
(a1) edge [loop above]  node {1}()
(e2) edge node[midway,right] {$e^2$}   (t2)
(c2) edge node[midway,right] {$c^2$}   (t2)
(a2) edge node[midway,right=2pt] {$h^2$}   (t2)
(e2) edge [loop above]  node {1}()
(c2) edge [loop above]  node {1}()
(a2) edge [loop above]  node {1}()
(a1) edge [<->,bend left]  node {1}(a2)
(c1) edge [<->,bend left=50]  node {1}(c2);
\end{tikzpicture}
\end{document}